This invention relates to methods and apparatus for data display and more particularly to the display of a measured variable in a moving strip chart format on a cathode ray tube.
Moving pen strip chart type recorders have been extensively used in industry to produce representations of measured dependent variables as a function of an independent variable, usually time. For example, in electric power generating plants moving pen recorders continuously monitor many of the parameters of power generation. The strip chart records provide an excellent visual representation of the operation of the plant at any given time. It has become common to record two or more measured variables on the same strip chart to provide a convenient way of directly comparing these measured variables one with the other to better determine their interrelationships. While the format of analog strip chart recorders has become widely accepted as a useful display medium, the extensive use of strip chart recorders has attendant problems.
Strip chart recorders produce a large amount of records and quite often it is only portions of these records produced in short time intervals which need to be studied. Further, it is only possible to record a limited number of measured variables on a single strip chart record without confusion. Often, two or more measured variables which the operator wishes to compare are not recorded on the same strip chart. In fact, if a large number, such as 100, measured variables are being recorded, the chances of two measured variables, which are desired for comparison, appearing on one strip chart are small.
Another disadvantage of the extensive use of strip chart recorders is caused by the necessary physical distance separating the recorders. In order to observe all of the measured variables, it is necessary for the operator to move from one strip chart recorder to another.
Finally, of course, strip chart recorders have the disadvantage of moving mechanical elements, such as the recorder pen. The movement of mechanical elements places an inherent limit on the speed of response, and accuracy of a moving pen strip chart recorder.
One example of a strip chart recorder is shown in U.S. Pat. No. 3,389,397--Lex et al.
Recently, cathode ray tubes have been extensively used for graphical data display. These devices avoid the problems of moving mechanical elements which are present in strip chart recorders. Refer for example to "A Color-Television Graph Plotter for Digital Computers," Claude A. Wiatrowski, Computer Design, April 1970, pages 133-136. This article describes the display of graphs in multiple colors on a cathode ray tube. "Drum and Scope Unit Plots Plant Variables," John Werme, Control Engineering, November 1964, Page 109, describes the display of a process variable on an oscilloscope type display screen. While graphic display systems of the foregoing type are extensively used in many applications, they are not suitable for use as a replacement for strip chart recorders in the monitoring of measured variables of industrial processes. As one reason, they are not capable of displaying the measured variables in a format to which operators have become accustomed and which format is particularly convenient for the monitoring of these measured variables.